Jeff I am no expert on TRS drives but they are pre Bravo and yes they use a transmission. TRS was a good drive in my opinion. TRS is a mercury drive and parts are still available but will be getting more difficult as time goes on to get easily. There are replacement drives also (Konrad for one) but they are not cheap.
Mercury went to the Bravo with a built in transmission for cost and mounting easily along with less weight. This allowed moving the motors back which is good or bad thing depending on the boat.
1980 hull make sure you check it for any water - wood damage. Drive it and like anything used plan on some needed $$$$ TLC. Boating gets in your blood and your pocket book! Buying used I hope you are a do it yourself person as repairs can get costly. Good Luck.
There are a couple of other boards you can search for info on also. Also the Mercuiserparts site. If you want PM me I can get you more places to look.
http://www.offshoreonly.com/forums/
http://www.speedwake.com/upload/index.php?s=
http://www.mercruiserparts.com/selec...?doc_nbr=76022